The present invention relates to fluid controllers for use in vehicle hydrostatic power steering systems, and more particularly, to such controllers which are to be used with fluid pressure operated steering actuators of the type which are likely to have noticeable leakage, from the inlet port to the outlet port of the actuator.
Although the present invention may be utilized in a hydrostatic power steering system in which the steering actuator is a linear cylinder (either single or double rod end), it is especially advantageous when utilized in a system in which the steering actuator is a rotary motor, and will be described in connection therewith. Examples of rotary motors which could be utilized as the steering actuator would be a gear motor, or a vane motor, or possibly a gerotor motor of the fixed axis type. Those skilled in the art will understand that the invention is especially advantageous when used with such a motor as the steering actuator for reasons which will become apparent subsequently.
In a typical hydrostatic power steering system, the steering actuator defines an inlet and an outlet which are connected to the control fluid ports of a fluid controller, which is also typically referred to as a steering control unit (SCU). The steering actuator receives at its inlet a flow of metered, pressurized fluid from the SCU, which results in an output motion which, in turn, results in appropriate movement of the steered wheels.
If the steering actuator is a rotary motor, there is likely to be the potential for a noticeable amount of internal leakage between the inlet and the outlet, effectively bypassing the rotating group. The amount of leakage which can or does occur during normal steering operations does not represent a significant problem in terms of the overall performance of the steering system.
However, it has been found that there may be a performance problem whenever one of the steered wheels of the vehicle engages an object, such as a curb, or for some other reason, the system is effectively steering xe2x80x9cagainst-the-stopsxe2x80x9d. Whenever a steer against-the-stops type of situation occurs, such that there is no rotary output from the rotating group of the motor, a substantial amount of the fluid communicated to the inlet of the motor will leak internally within the motor, bypassing the rotating group, and flowing to the outlet port.
One result of such internal leakage within the steering actuator is that the SCU continues to communicate metered, pressurized fluid to the inlet of the actuator. Thus, even though no additional turning motion is being transmitted to the steered wheels, the operator is still able to rotate the steering wheel. Such ongoing ability to rotate the steering wheel, without any corresponding change in the position of the steered wheels, appears to the vehicle operator as steering wheel xe2x80x9cslipxe2x80x9d which is considered a very undesirable characteristic of a steering system. Typically, vehicle manufacturers specify a maximum, permissible steering wheel slip, generally in terms of the maximum permissible number of rotations of the steering wheel over a given time period. For example, a typical specification for wheel slip would be somewhere in the range of about two to about five revolutions per minute, as a maximum, permissible amount of slip.
As is well know to those skilled in the art of fluid controllers, a typical fluid controller of the type to which the present invention relates includes some sort of controller valving defining a neutral position (when there is no steering input), a normal operating position (when normal steering is occurring) and a maximum displacement position, i.e., the maximum opening (flow area) of the valving. Whenever the steering system is in a steer against-the-stops type of situation, the controller valving is typically displaced to the maximum displacement condition. The conventional neutral return spring, which is present in most such fluid controllers, tends to return the valving toward the neutral position, but is at its fully deflected condition when the valving is in the maximum displacement condition.
Fluid controllers of the type which can utilize the present invention typically include some sort of fluid actuated arrangement for imparting follow-up movement to the controller valving, tending to return the valving from its normal operating position toward its neutral position. In the fluid controllers produced and sold by the assignee of the present invention, the fluid actuated arrangement is a fluid meter which comprises a gerotor gear set. The gerotor gear set includes an internally toothed ring and an externally toothed star, disposed eccentrically within the ring. One possible solution to the apparent slip problem discussed above is to increase the tip clearance of the teeth in the gerotor gear set, thus communicating some flow through the fluid meter to the control fluid port, to compensate for the leakage within the actuator. Unfortunately, such an increase in the tip clearance within the gerotor gear set has been found to permit a condition known as xe2x80x9cfeed-throughxe2x80x9d in which there is a flow of fluid through the fluid meter even at times when such is not desired.
Accordingly, it is an object of the present invention to provide an improved hydrostatic power steering system of the type described above which overcomes, in the situation described, the apparent wheel slip of the fluid controller.
It is a more specific object of the present invention to provide an improved fluid controller for use in such a system in which the fluid controller is able effectively to compensate for the leakage which is occurring at the steering actuator.
The above and other objects of the invention are accomplished by the provision of a fluid controller operable to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated device having an inlet and an outlet and defining a fluid leakage path therebetween. The fluid controller is of the type including a housing defining an inlet port for connection to the source of pressurized fluid, a return port for connection to a system reservoir, and a control port for connection to the inlet of the fluid pressure operated device. The controller includes valving disposed in the housing and defining a neutral position, a normal operating position, and a maximum displacement position. The housing and the valving cooperate to define a main fluid path providing fluid communication between the inlet port and the control port when the valving is in the normal operating position. The controller includes a fluid actuated means for imparting follow up movement to the valving, tending to return the valving from its normal operating position toward the neutral position, the follow up movement being proportional to the volume of fluid flow through the main fluid path.
The improved fluid controller is characterized by the valving defining a fluid bleed passage having an upstream portion in fluid communication with the main fluid path at a location upstream of the fluid actuated means, and a downstream portion in fluid communication with the main fluid path downstream of the fluid actuated means. The fluid bleed passage includes a variable bleed orifice having a substantially zero flow area when the valving is in the neutral position and in the normal operating position. The variable bleed orifice begins to open as the valving approaches the maximum displacement position.
Therefore, in accordance with the present invention, whenever the steering system is in a steer against-the-stops situation, and the valving is in the maximum displacement position, a small amount of fluid is communicated through the fluid bleed passage of the fluid controller to compensate for the amount of leakage anticipated through the steering actuator.